The trucking industry accounts for more than 5% of the U.S. GDP and is comprised of more that 500,000 for-hire, private and government fleets, including owner operators. It is a barometer of the US economy representing nearly 70% of the tonnage carried by all modes of domestic freight transportation, including manufactured and retail goods. This industry is powered almost exclusively by diesel engines (compressive ignition engines), which are characterized by high torque developed at low rpm and 25% greater thermodynamic efficiency compared to spark ignition (gasoline) engines. As a result of the 2007 EPA mandated emissions reductions in oxides of nitrogen (NOx) and diesel particulate matter (DPM or soot), diesel-powered vehicles are now required to be fitted with diesel oxidation catalysts (DOC) or some form of catalytic converter and to burn ultra low sulfur diesel fuel, ULSD, (<15 ppm S). These and other technologies such as EGR (emissions gas recirculation) are necessary to meet the EPA mandated emissions standards. The ULSD requirement is a consequence of sulfur poisoning of the precious metals on the DOC by high sulfur levels. This legislation has far ranging consequences, as (on road) diesel fuel in the US is consumed at a prodigious rate, 650M gal/week, which is second only to that of gasoline (1300 M gal/wk).
It is estimated that the imputed costs of the EPA mandates will add approximately $0.39 to the cost of one gallon of diesel fuel. This is factored into the following components: increased engine costs ($0.11/gal), particle trap maintenance ($0.05/gal), reduced fuel economy ($0.09/gal), increase in ULSD ($0.06/gal), and lower ULSD fuel energy content ($0.08/gal).
Clearly any technology that could provide a reduction in DPM and other emissions, simultaneously with an increase in fuel economy (as measured by an increase in miles-per-gallon) would be perceived as a tremendous financial and environmental benefit.
Diesel fuel additives, in particular, those that include to inorganic metal and metal oxide materials as opposed to organic materials, offer the promise of reduced DPM and improved fuel economy.
Kracklaurer, U.S. Pat. No. 4,389,220, the disclosure of which is incorporated herein by reference, describes a method of conditioning diesel engines in which a diesel engine is operated on a diesel fuel containing from about 20-30 ppm of dicyclopentadienyl iron for a period of time sufficient to eliminate carbon deposits from the combustion surfaces of the engine and to deposit a layer of iron oxide on the combustion surfaces, which layer is effective to prevent further buildup of carbon deposits. Subsequently, the diesel engine is operated on a maintenance concentration of from about 10-15 ppm of dicyclopentadienyl iron or an equivalent amount of a derivative thereof on a continuous basis. The maintenance concentration is effective to maintain the catalytic iron oxide layer on the combustion surfaces but insufficient to decrease timing delay in the engine. The added dicyclopentadienyl iron may produce iron oxide on the engine cylinder surface (Fe2O3), which reacts with carbon deposits (soot) to form Fe and CO2, thereby removing the deposits. However, this method may accelerate the aging of the engine by formation of rust.
Valentine, et al., U.S. Patent Appl. Publ. No. 2003/0148235, the disclosure of which is incorporated herein by reference, describe specific bimetallic or trimetallic fuel-borne catalysts for increasing the fuel combustion efficiency. The catalysts reduce fouling of heat transfer surfaces by unburned carbon while limiting the amount of secondary additive ash, which may itself cause overloading of particulate collector devices or emissions of toxic ultra fine particles when used in forms and quantities typically employed. By utilizing a fuel containing a fuel-soluble catalyst comprised of platinum and at least one additional metal comprising cerium and/or iron, production of pollutants of the type generated by incomplete combustion is reduced. Ultra low levels of nontoxic metal combustion catalysts can be employed for improved heat recovery and lower emissions of regulated pollutants. However, fuel additives of this type, in addition to using the rare and expensive metals such as platinum, can require several months before the engine is “conditioned”. By “conditioned” is meant that all the benefits of the additive are not obtained until the engine has been operated with the catalyst for a period of tune. Initial conditioning may require 45 days and optimal benefits may not be obtained until 60-90 days. Additionally, free metal may be discharged from the exhaust system into the atmosphere, where it may subsequently react with living organisms.
Cerium dioxide is widely used as a catalyst in converters for the elimination of toxic exhaust emission gases and the reduction in particulate emissions in diesel powered vehicles. Within the catalytic converter, the cerium dioxide can act as a chemically active component, acting to release oxygen in the presence of reductive gases, as well as to remove oxygen by interaction with oxidizing species.
Cerium dioxide may store and release oxygen by the reversible process shown in equation 1.CeO2←→CeO2-x+x/2O2  (eq. 1)This process is referred to as the oxygen storage capability (OSC) of ceria. Here ceria acts as an oxygen storage buffer (much like a pH buffer), releasing oxygen in spatial regions where the oxygen concentration or pressure is low and absorbing oxygen in spatial regions where the oxygen pressure is high. When x=0.5, ceria is effectively fully reduced to Ce2O3, and the maximum theoretical OSC is 1452 micromoles of O2 per gram of ceria. The redox potential between the Ce3+ and Ce4+ ions lies between 1.3 and 1.8V and is highly dependent upon the anionic groups present and the chemical environment (CERIUM: A Guide to its Role in Chemical Technology, 1992 by Molycorp, Library of Congress Catalog Card Number 92-93444)). This allows the described forward and backward reactions to easily occur in exhaust gases near the stoichiometric ratio of required oxygen (15:1). Cerium dioxide may provide oxygen for the oxidation of CO or hydrocarbons in an oxygen-starved environment, or conversely may absorb oxygen for reducing the levels of nitrogen oxides (NOx) in an oxygen-rich environment. Similar catalytic activity may also occur when cerium dioxide is added as an additive to fuel, for example, diesel or gasoline. However, for this effect to be useful, the cerium dioxide must be of a particle size small enough, i.e., nanoparticulate (less than 100 nm), to remain suspended by Brownian motion in the fuel and not settle out. In addition, as catalytic effects depend on surface area, the small particle size renders the nanocrystalline material more effective as a catalyst. The incorporation of cerium dioxide in fuel serves not only to act as a catalyst to reduce toxic exhaust gases produced by fuel combustion, for example, by the “water gas shift reaction”CO+H2O→CO2+H2,but also to facilitate the burning off of particulates that accumulate in the particulate traps typically used with diesel engines.
As already noted, cerium dioxide nanoparticles are particles having a mean diameter of less than 100 nm. For the purposes of this disclosure, unless otherwise stated, the diameter of a nanoparticle refers to its hydrodynamic diameter, which is the diameter determined by dynamic light scattering technique and includes molecular adsorbates and the accompanying solvation shell of the particle. Alternatively, the geometric particle diameter can be estimated using transmission electron micrography (TEM).
Vehicle on-board dosing systems that dispense cerium dioxide into the fuel before it enters the engine are known, but such systems are complicated and require extensive electronic control to feed the appropriate amount of additive to the fuel. To avoid such complex on-board systems, cerium dioxide nanoparticles can also be added to fuel at an earlier stage to achieve improved fuel efficiency. They can, for example, be incorporated at the refinery, typically along with processing additives such as, for example, cetane improvers or lubricity agents, or added at a fuel distribution tank farm.
Cerium dioxide nanoparticles can also be added at a fuel distribution center by rack injection into large (˜100,000 gal) volumes of fuel, or at a smaller fuel company depot, which would allow customization according to specified individual requirements. In addition, the cerium dioxide may be added at a filling station during delivery of fuel to a vehicle, which would have the potential advantage of improved stabilization of the particle dispersion.
Cerium nanoparticles may form a ceramic layer on the engine cylinders and internal moving parts, thereby essentially turning the engine into a catalytic device. Alternatively, they may be recycled in the lubrication oil where they accumulate. Their catalytic efficiency derives from the fact that they provide a source of oxygen atoms during combustion by undergoing reduction according to the equation (1); however, an induction period of several months is usually required before their mpg benefit is observed. This ultimately results in better fuel combustion and reduced levels of particulate material emissions. Additionally, when used as a fuel additive, these nanoparticles may provide improved engine performance by reducing engine friction. As an alternative mode of introduction, cerium dioxide nanoparticles can be added to the lube oil and act as a lubricity enhancing agent to reduce internal friction. This will also improve fuel efficiency.
The following publications, the disclosures all of which are incorporated herein by reference, describe fuel additives containing cerium oxidic compounds.
Hawkins et al., U.S. Pat. No. 5,449,387, discloses a cerium (IV) oxidic compound having the formula:(H2O)p[CeO(A)2(AH)n]m 
in which the radicals A, which are the same or different, are each an anion of an organic oxyacid AH having a pKa greater than 1, p is an integer ranging from 0 to 5, n is a number ranging from 0 to 2, and m is an integer ranging from 1 to 12. The organic oxyacid is preferably a carboxylic acid, more preferably, a C2-C20 monocarboxylic acid or a C4-C12 dicarboxylic acid. The cerium-containing compounds can be employed as catalysts for the combustion of hydrocarbon fuels.
Valentine et al., U.S. Pat. No. 7,063,729, discloses a low-emissions diesel fuel that includes a bimetallic, fuel-soluble platinum group metal and cerium catalyst, the cerium being provided as a fuel-soluble hydroxyl oleate propionate complex.
Chopin et al., U.S. Pat. No. 6,210,451, discloses a petroleum-based fuel that includes a stable organic sol that comprises particles of cerium dioxide in the form of agglomerates of crystallites (preferred size 3-4 nm), an amphiphilic acid system containing at least one acid whose total number of carbons is at least 10, and an organic diluent medium. The controlled particle size is no greater than 200 nm.
Birchem et al., U.S. Pat. No. 6,136,048, discloses an adjuvant for diesel engine fuels that includes a sol comprising particles of oxygenated compound having a d90 no greater than 20 nm, an amphiphilic acid system, and a diluent. The oxygenated metal compound particles are prepared from the reaction in solution of a rare earth salt such as a cerium salt with a basic medium, followed by recovery of the formed precipitate by atomization or freeze drying.
Lemaire et al., U.S. Pat. No. 6,093,223, discloses a process for producing aggregates of ceric oxide crystallites by burning a hydrocarbon fuel in the presence of at least one cerium compound. The soot contains at least 0.1 wt. % of ceric oxide crystallite aggregates, the largest particle size being 50-10,000 angstroms, the crystallite size being 50-250 angstroms, and the soot having an ignition temperature of less than 400° C.
Hazarika et al., U.S. Pat. No. 7,195,653 B2, discloses a method of improving fuel efficiency and/or reducing fuel emissions of a fuel burning apparatus, the method comprising dispersing at least one particulate lanthanide oxide, particularly cerium dioxide, in the fuel at 1 to 10 ppm, either as a tablet, a capsule a powder or liquid fuel additive wherein the particulate lanthanum oxide is coated with a surfactant selected from the group consisting of alkyl carboxylic anhydrides and esters having an HLB of 7 or less. The preferred coating is dodecyl succinic anhydride.
Collier et al., U.S. Patent Appl. Publ. No. 2003/0182848, discloses a diesel fuel composition that improves the performance of diesel fuel particulate traps and contains a combination of 1-25 ppm of metal in the form of a metal salt additive and 100-500 ppm of an oil-soluble nitrogen-containing ashless detergent additive. The metal may be an alkali metal, an alkaline earth metal, a metal of Group IVB, VIIB, VIIIB, IB, IIB, or any of the rare earth metals having atomic numbers 57-71, especially cerium, or mixtures of any of the foregoing metals.
Collier et al., U.S. Patent Appl. Publ. No. 2003/0221362, discloses a fuel additive composition for a diesel engine equipped with a particulate trap, the composition comprising a hydrocarbon solvent and an oil-soluble metal carboxylate or metal complex derived from a carboxylic acid containing not more than 125 carbon atoms. The metal may be an alkali metal, an alkaline earth metal, a metal of Group IVB, VIIB, VIIIB, IB, IIB, or a rare earth metal, including cerium, or mixtures of any of the foregoing metals.
Caprotti et al., U.S. Patent Appl. Publ. No. 2004/0035045, discloses a fuel additive composition for a diesel engine equipped with a particulate trap. The composition comprises an oil-soluble or oil-dispersible metal salt of an acidic organic compound and a stoichiometric excess of metal. When added to the fuel, the composition provides 1-25 ppm of metal, which is selected from the group consisting of Ca, Fe, Mg, Sr, Ti, Zr, Mn, Zn, and Ce.
Caprotti et al., U.S. Patent Appl. Publ. No. 2005/0060929, discloses a diesel fuel composition stabilized against phase separation that contains a colloidally dispersed or solubilized metal catalyst compound and 5-1000 ppm of a stabilizer that is an organic compound having a lipophilic hydrocarbyl chain attached to at least two polar groups, at least one of which is a carboxylic acid or carboxylate group. The metal catalyst compound comprises one or more organic or inorganic compounds or complexes of Ce, Fe, Ca, Mg, Sr, Na, Mn, Pt, or mixtures thereof.
Caprotti et al., U.S. Patent Appl. Publ. No. 2006/0000140, discloses a fuel additive composition that comprises at least one colloidal metal compound or species and a stabilizer component that is the condensation product of an aldehyde or ketone and a compound comprising one or more aromatic moieties containing a hydroxyl substituent and a further substituent chosen from among hydrocarbyl, —COOR, or —COR, R being hydrogen or hydrocarbyl. The colloidal metal compound preferably comprises at least one metal oxide, preferred oxides being iron oxide, cerium dioxide, or cerium-doped iron oxide.
Scattergood, International Publ. No. WO 2004/065529, discloses a method for improving the fuel efficiency of fuel for an internal combustion engine that comprises adding to the fuel cerium dioxide and/or doped cerium dioxide and, optionally, one or more fuel additives.
Anderson et al., International Publ. No. WO 2005/012465, discloses a method for improving the fuel efficiency of a fuel for an internal combustion engine that comprises lubricating oil and gasoline, the method comprising adding cerium dioxide and/or doped cerium dioxide to the lubricating oil or the gasoline.
Cerium-containing nanoparticles can be prepared by a variety of techniques known in the art. Regardless of whether the synthesized nanoparticles are made in a hydrophilic or hydrophobic medium, the particles normally require a stabilizer to prevent undesirable agglomeration. The following publications, the disclosures all of which are incorporated herein by reference, describe some of these synthetic techniques.
Chane-Ching et al., U.S. Pat. No. 6,271,269, discloses a process for preparing storage-stable organic sols that comprises: reacting a base reactant with an aqueous solution of the salt of an acidic metal cation to form an aqueous colloidal dispersion containing excess hydroxyl ions; contacting the aqueous colloidal dispersion with an organic phase comprising an organic liquid medium and an organic acid; and separating the resulting aqueous/organic phase mixture into an aqueous phase and a product organic phase. Preferred metal cations are cerium and iron cations. The colloidal particulates have hydrodynamic diameters in the range of 5-20 nanometers.
Chane-Ching, U.S. Pat. No. 6,649,156, discloses an organic sol containing cerium dioxide particles that are made by a thermal hydrolysis process; an organic liquid phase; and at least one amphiphilic compounds chosen from polyoxyethylenated alkyl ethers of carboxylic acids, polyoxyethylenated alkyl ether phosphates, dialkyl sulfosuccinates, and quaternary ammonium compounds. The water content of the sols may not be more than 1%. The mean crystallite size is about 5 nm, while the particle agglomerates of these crystallites range in size from 200 to 10 nm.
Chane-Ching, U.S. Pat. No. 7,008,965, discloses an aqueous colloidal dispersion of a compound of cerium and at least one other metal, the dispersion having a conductivity of at most 5 mS/cm and a pH between 5 and 8.
Chane-Ching, U.S. Patent Appl. Publ. No. 2004/0029978 (abandoned Dec. 7, 2005), discloses a surfactant formed from at least one nanoparticle that has amphiphilic characteristics and is based on a metal oxide, hydroxide and/or oxyhydroxide, on the surface of which organic chains with hydrophobic characteristics are bonded. The metal is preferably selected from among cerium, aluminum, titanium or silicon, and the alkyl chain comprises 6-30 carbon atoms, or polyoxyethylene monoalkyl ethers of which the alkyl chain comprises 8-30 carbon atoms and the polyoxyethylene part comprises 1-10 oxyethylene groups. The particle is an isotopic or spherical particle having an average diameter of 2-40 nm.
Blanchard et al., U.S. Patent Appl. Publ. No. 2006/0005465, discloses an organic colloidal dispersion comprising: particles of at least one compound based on at least one rare earth, at least one acid, and at least one diluent, wherein at least 90% of the particles are monocrystalline. Example 1 describes the preparation of a cerium dioxide colloidal solution from cerium acetate and an organic phase that includes Isopar hydrocarbon mixture and isostearic acid. The resulting cerium dioxide particles had a d50 of 2.5 nm, and the size of 80% of the particles was in the range of 1-4 nm.
Zhou et al., U.S. Pat. No. 7,025,943, discloses a method for producing cerium dioxide crystals that comprises: mixing a first solution of a water-soluble cerium salt with a second solution of alkali metal or ammonium hydroxide; agitating the resulting reactant solution under turbulent flow conditions while concomitantly passing gaseous oxygen through the solution; and precipitating cerium dioxide particles having a dominant particle size within the range of 3-100 nm. In Example 1, the particle size is stated to be around 3-5 nm. No mention is made of a stabilizing agent and it is anticipated that the sols will eventually agglomerate and settle.
Sandford et al., WO 2008/002223 A2, describe an aqueous precipitation technique that produces cerium dioxide directly without subsequent calcination. Cerous+3 cation is oxidized to ceric+4 slowly by nitrate ion, and a stable non-agglomerated sol of 11 nm crystallite size (and approximately equal grain size) is obtained when acetic acid is used as a stabilizer. Interestingly, EDTA and citric acid produce grains with crystallite sizes on the order of several hundred nanometers.
Woodhead, James, L. U.S. Pat. No. 4,231,893, teaches the preparation of an aqueous dispersion of ceria by the acid treatment of Ce(OH)4 which has been obtained from the peroxide treatment of Ce+0 in base. No sizing data are provided and at the required pH for stabilization, 1.5, the likely stabilizer is NO3− anion.
Noh et al., U.S. Patent Appl. Publ. No. 2004/0241070, discloses a method for preparing single crystalline cerium dioxide nanopowder comprising: preparing cerium hydroxide by precipitating a cerium salt in the presence of a solvent mixture of organic solvent and water, preferably in a ratio of about 0.1:1 to about 5:1 by weight; and hydrothermally reacting the cerium hydroxide. The nanopowder has a particle size of about 30-300 nm.
Chan, U.S. Patent Appl. Publ. No. 2005/0031517, discloses a method for preparing cerium dioxide nanoparticles that comprises: rapidly mixing an aqueous solution of cerium nitrate with aqueous hexamethylenetetramine, the temperature being maintained at a temperature no higher than about 320° K while nanoparticles form in the resulting mixture; and separating the formed nanoparticles. The mixing apparatus preferably comprises a mechanical stirrer and a centrifuge. In the illustrative example, the prepared cerium dioxide particles are reported to have a diameter of about 12 nm.
Ying et al., U.S. Pat. Nos. 6,413,489 and 6,869,584, disclose the synthesis by a reverse micelle technique of nanoparticles that are free of agglomeration and have a particle size of less than 100 nm and a surface area of at least 20 m2/gm. The method comprises introducing a ceramic precursor that includes barium alkoxide and aluminum alkoxide in the presence of a reverse emulsion.
A related publication, Ying et al., U.S. Patent Appl. Publ. No. 2005/0152832, discloses the synthesis, by a reverse micelle technique within an emulsion having a 1-40% water content, of nanoparticles that are free of agglomeration and have a particle size of less than 100 nm. The nanoparticles are preferably metal oxide particles, which can be used to oxidize hydrocarbons.
Hanawa et al., U.S. Pat. No. 5,938,837, discloses a method for preparing cerium dioxide particles, intended primarily for use as a polishing agent, that comprises mixing, with stirring, an aqueous solution of cerous nitrate with a base, preferably aqueous ammonia, in such a mixing ratio that the pH value of the mixture ranges from 5 to 10, preferably 7 to 9, then rapidly heating the resulting mixture to a temperature of 70-100° C., and maturing the mixture of cerous nitrate with a base at that temperature to form the grains. The product grains are uniform in size and shape and have an average particle size of 10-80 nm, preferably 20-60 nm.
European Patent Application EP 0208580, published 14 Jan. 1987, inventor Chane-Ching, applicant Rhone Poulenc, discloses a cerium (IV) compound corresponding to the general formulaCe(M)x(OH)y(NO3)2 
wherein M represents an alkali metal or quaternary ammonium radical, x is between 0.01 and 0.2, y is such that y=4−z+x, and z is between 0.4 and 0.7. A process for preparing a colloidal dispersion of the cerium (IV) compound produces particles with a hydrodynamic diameter between about 1 nm and about 60 nm, suitably between about 1 nm and about 10 nm, and desirably between about 3 nm and 8 nm.
S. Sathyamurthy et al., Nano Technology 16, (2005), pp 1960-1964, describes the reverse micellar synthesis of CeO2 from cerium nitrate, using sodium hydroxide as the precipitating agent and n-octane containing the surfactant cetyltrimethylammonium bromide (CTAB) and the cosurfactant 1-butanol as the oil phase. The resulting polyhedral particles had an average size of 3.7 nm, and showed agglomeration when removed from their protective reversed micellar structure. Additionally, the reaction would be expected to proceed in low yield (for reactants A and B there are as many AB collisions resulting in product as AA and BB non productive collisions).
Seal et al., Journal of Nano Particle Research, (2002), 4, pp 433-448, describes the preparation from cerium nitrate and ammonium hydroxide of nanocrystalline ceria particles for a high-temperature oxidation-resistant coating using an aqueous microemulsion system containing AOT as the surfactant and toluene as the oil phase. The ceria nanoparticles formed in the upper oil phase of the reaction mixture had a particle size of 5 nm.
Seal et al., U.S. Pat. No. 7,419,516, the disclosure of which is incorporated herein by reference, describes the use of rare earth metal oxide, preferably ceria, nanoparticles as fuel additives for reducing soot. The particles, which are prepared by a reverse micelle process using toluene as the oil phase and AOT as the surfactant, have diameters in the range of about 2-7 nm, the mean being about 5 nm.
Pang et al., J. Mater. Chem., 12 (2002), pp 3699-3704, prepared Al2O3 nanoparticles by a water-in-oil microemulsion method, using an oil phase containing cyclohexane and the non-ionic surfactant Triton X-114, and an aqueous phase containing 1.0 M AlClO3. The resulting Al2O3 particles, which had a particle size of 5-15 nm, appeared to be distinctly different from the hollow ball-shaped particles of submicron size made by a direct precipitation process.
Cuif et al, U.S. Pat. No. 6,133,194, the disclosure of which is incorporated herein by reference, describes a process that comprises reacting a metal salt solution containing cerium, zirconium, or a mixture thereof, a base, optionally an oxidizing agent, and an additive selected from the group consisting of anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids, and carboxylate salts, thereby forming a product. The product is subsequently calcined at temperatures greater than 500° C. (which would effectively carbonize the claimed surfactants).
It should be appreciated that, while many authors claim ceria nanoparticles well below 5 nm, no X-ray or electron diffraction data have been presented to unequivocally establish that the grains are indeed cubic CeO2 and not hexagonal or cubic Ce2O3. There is substantial doubt that cubic CeO2 is thermodynamically stable at very small grain sizes, and that the grains are, in fact, the reduced and more stable hexagonal Ce2O3 form. S. Tsunekawa, R, Sivamohan, S. Ito, A. Kasuya and T. Fukada in Nanostructured Materials, vol 11, no. 1, pp 141-147 (1999)
“Structural Study on Monosize CeO2-x Nanoparticles” in particular casts doubt upon the existence of CeO2 at or below 1.5 nm.
Additional evidence for the existence of Ce3+ (and by extension Ce2O3) at very small grain diameters comes from the work of Desphande et al. in Applied Physics Letters 87, 133113 (2005) “Size Dependency Variation in Lattice Parameter and Valency States in Nano Crystalline Cerium Oxide”, who found a log linear relationship between the change in lattice constant,Δa=a−a0(a0=5.43 Å in CeO2) and the crystal diameter, D, as follows:log Δa=−0.4763 log D−1.5029  (Eq. 2)
Thus, a grain diameter of 10 nm will experience a lattice strain or change in the lattice constant of 0.0103 Å or 1.91%, whereas a 1 nm diameter grain will experience a change of 0.031 Å or 5.73 percent.
The extent to which CeO2 can act as a catalytic oxygen storage material, described by equation 1, is governed in part by the CeO2 particle size. At 20 nm particle sizes and below, the lattice parameter increases dramatically with decreasing crystallite size (up to 0.45% at 6 nm, see for example Zhang, et al., Applied Physics Letters, 80 1, 127 (2002)). The associated size-induced lattice strain is accompanied by an increase in surface oxygen vacancies that results in enhanced catalytic activity. This inverse size-dependent activity provides not only for more efficient fuel cells, but better oxidative properties when used in the combustion of petroleum fuels.
As described previously, various methods and apparatus have been reported for preparing cerium nanoparticles, including those described by Chane-Ching, et al., U.S. Pat. No. 5,017,352; Hanawa, et al., U.S. Pat. No. 5,938,837; Melard, et al., U.S. Pat. No. 4,786,325; Chopin, et al., U.S. Pat. No. 5,712,218; Chan, U.S. Patent Appl. Publ. No. 2005/0031517; and Zhou, et al., U.S. Pat. No. 7,025,943, the disclosures of which are incorporated herein by reference. However, current methods do not allow the economical, facile (i.e. non-calcined) and unambiguous preparation of cubic CeO2 nanoparticles in high yield, in a short period of time at very high suspension densities (greater than 0.5 molal, i.e., 9 wt. % that are sufficiently small in size (less than 5 nm in mean geometric diameter), uniform in size frequency distribution (coefficient of variation [COV] of less than 25%, where COV is the standard deviation divided by the mean diameter), and stable for many desirable applications. Additionally, it would be very desirable to produce particles that are crystalline, ie, a single crystal rather than an agglomeration of crystallites of various sizes such as are taught in the above mentioned art and technical literature.
Although substantially pure cerium dioxide nanoparticles are beneficially included in fuel additives, it may be of further benefit to use cerium dioxide doped with components that result in the formation of additional oxygen vacancies being formed (Eq. 1). For this to occur, the dopant ion should be divalent or trivalent, i.e., a divalent or trivalent ion of an element that is a rare earth metal, a transition metal or a metal of Group IIA, IIIB, VB, or VIB of the Periodic Table. The requirement for crystal charge neutrality using these lower valence cations will drive Eq. 1 to the right, i.e., higher extent of oxygen vacancy formation. Metal dopant ions with smaller ionic radii than Ce+4 (0.97 Å in an octahedral configuration) will also aid in oxygen vacancy formation since this process reduces two adjacent Ce+4 ions (one surface and one subsurface) to Ce+3 whose resultant larger ionic radius, 1.143 Å, expands the lattice, thereby causing lattice strain, Thus substituting Zr+4 (ionic radius 0.84 Å) or Cu+2 (ionic radius of six coordinate octahedral configuration is 0.73 Å, four coordinate tetrahedral 0.57 Å) will relieve some of this lattice strain. Additionally, Zr allows the formation of two adjacent surface Ce+3 species (rather than one surface and one subsurface), which may be important for very small particles where approximately 50% of the ions are surface ions. One can thus appreciate that substitutional ion doping is preferred to interstitial ion doping, where the dopants occupy spaces between the normal lattice positions.
For the purposes of this discussion, we need to distinguish what is meant by doping as opposed to a lattice engineered crystal. In semiconductor physics, the word doping refers to n or p type impurities present in the parts-per-million range. We use the word doped crystal to refer to a crystal that has on or more metal dopant ions present in concentrations less than 2 mole percent (20,000 ppm). A lattice engineered crystal, on the other hand, can have one or more metal dopant ions present in the CeO2 crystal at concentrations greater than 20,000 ppm up to 800,000 ppm (or 80% of the cerium sub-lattice). Thus a lattice engineered cerium dioxide crystal could have cerium present as the minor metal component.
Doping of cerium dioxide with metal ions to improve ionic transport, reaction efficiency and other properties is described in, for example, “Doped Ceria as a Solid Oxide Electrolyte, H. L. Tuller and A. S. Nowick in J. Electrochem Soc., 1975, 122(2), 255; “Point Defect Analysis and Microstructural Effects in Pure and Donor Doped Ceria”, M. R. DeGuire, et. al., Solid State Ionics, 1992, 52, 155; and “Studies on Cu/CeO2: A New NO Reduction Catalyst” Parthasarathi Bera, S. T. Aruna, K. C. Patil, and M. S. Hegde in Journal of Catalysis, 186, 36-44 (1999) and. The resultant dopant effects on the electronic and oxygen diffusion properties are described by Trovarelli, Catalysis by Ceria and Related Materials, Catalytic Science Series, World Scientific Publishing Co., 37-46 (2002) and references cited therein.
Trovarelli et al. in Catalysis Today, 43 (1998), 79-88, discuss the preparation of ceria-zirconia mixed oxides of fairly good compositional homogeneity using a surfactant-assisted approach. High specific surface areas, 230 m2/gm, are obtained after calcination of compositions at 723° K; however, sintering occurs at 1173° K as the specific surface area drops to 40 m2/gm (˜20 nm diameter).
Pulsed neutron diffraction techniques were used by E. Mamontov, et al. J. Phys. Chem. B 2000, 104, 1110-1116 to study ceria and ceria-zirconia solid solutions. These studies established for the first time the correlation between the concentration of vacancy-interstitial oxygen defects and the oxygen storage capability. They postulate that the preservation of oxygen defects, which Zr aids, is necessary to ameliorate the degradation of OSC as a function of thermal aging. ZrO2 was present at 30.5 mole %, and the calcined particles had a diameter of approximately 40 nm, based upon BET surface area measurements.
Z. Yang et al. in Journal of Chemical Physics, (2006) 124 (22), 224704/(1-7) calculated from first principles, using density functional theory, that an oxygen vacancy is most easily created close to a Zr center, and therefore these centers serve as a nucleation site for vacancy clustering. The released oxygen donates two electrons to Ce+4 centers neighboring the vacancy, resulting in two Ce+3 centers.
R. Wang et al. in J. Chem. Phys. B, 2006, 110, 18278-18285 examined the spatial distribution of Zr in Ce0.5Zr0.5O2 produced by a spray freezing technique, followed by calcination. They find that particle nanoscale heterogeneity, as characterized by Ce-rich cores and Zr-rich shells in particles in the 5.4 to 25 nm particle size range, is associated with more redox active materials. This finding implies that a homogeneous distribution of Zr and Ce results in decreased activity and is therefore not preferred.
S. Bedrane et al. in Catalysis Today, 75, 1-4, 401-405 July 2002, measured the oxygen storage capability (Eq 1.) of 11 precious and noble metal (PM=Rh, Pt, Rd, Ru, and Ir) doped ceria (CeO2) and ceria-zirconia (Ce0.63Zr0.37O2) compositions. They observe a leveling effect in which the Ce—Zr materials have an OSC that is nearly independent of PM concentration and is 2 to 4 times as great as the PM-loaded Ce-only materials.
H. Sparks et al. of Nanophase Technologies, Corp., using vapor phase synthesis, manufactured ceria mixed with rare earth oxide nanomaterials (Mat. Res. Soc. Symp. Proc., Vol 788, 2004). They observe enhanced thermal stability of nanocrystalline particle size and an increase in OSC for the Zr-doped ceria (1:1); however further addition of La or Pr to the Zr composition, while better than ceria itself, was poorer than just the zirconium ceria combination. One can infer, from the reported specific surface areas, a particle size of 10 nm at 600° C., which increases to 40 nm at 1050° C.
The catalytic effects of Zr and Fe doped CeO2 in the combustion of diesel soot were examined by Aneggi et al. in Catalysis Today, 114, (2006), 40-47. They reiterated the fact that Zr enhances the thermal stability and OSC of pure ceria and found that Fe2O3 gave better fresh results, but there was a net loss of activity after calcination. A very systematic level series in Zr and Zr with Fe was examined, including crystallographic data on these calcined particles that were approximately 21 nm. They determined a nanoparticle specific area threshold, 35 m2/gm (corresponding to a diameter of less than 24 nm), in which the fresh versus aged activity was unchanged.
Copper-based catalytic systems have also received much attention. In a very thorough structural analysis of 3 and 5 atom percent Cu/CeO2, M. S. Hegde et al., Chem. Mater. 2002, 14, 3591-3601, demonstrated that Cu forms a distinct solid solution of Ce1-xCuxO2 with no discrete CuO phase. In these 50 nm agglomerated grains produced by combustion synthesis, the Cu is in the +2 state and is much more catalytically active than Cu in CuO. Furthermore, the oxygen ion vacancy is created around the Cu+2 cation.
A. Martinex-Arias et al. in J. Phys. Chem. B, 2005, 109, 19595-19603, found that the reduction of Ce1-xCuxO2 fluorite type nanoparticles (x=0.05, 0.1, and 0.2) was reversible and that the oxidation state of Cu was higher than its normal states (+1 or +2). The dopant induced a large lattice strain in these ˜6 nm particles in the oxide sub-lattice, which favored the formation of oxygen vacancies. A reverse microemulsion method followed by calcination at 500° C. was used to prepare these materials.
Iron is another metal ion that has imbued CeO2 nanoparticles with enhanced catalytic activity. I. Melian-Cabrera et al. in Journal of Catalysis, 239, 2006, 340-346, report enhanced activity (relative to the undoped materials) and optimal catalytic destruction of N2O, an oxygen-limited reaction, with a 50/50 composition of cerium and iron oxide. The Fe-doped ceria is made by a co-precipitation method that produces particles in the 30 nm diameter range.
T. Campenon and colleagues in SAE special publication SP 2004, SP-1860, “Diesel Exhaust Emission Control” use iron doped ceria to control the ash buildup in diesel particulate filters.
R. Hu and colleagues in Shiyou Huagong (2006), 35(4), 319-323 examined Fe-doped cerium dioxide made by a solid phase milling technique, followed by calcination at various elevated temperatures. Iron doping improved the catalytic activity with respect to the combustion of methane while simultaneously decreasing particle size.
Illustrative Examples 9 and 10 of U.S. Patent Appl. Publ. No. 2005/0152832 describe the preparation of, respectively, cerium-doped and cerium-coated barium hexaaluminate particles. Example 13 describes the oxidation of methane with the cerium-coated particles.
Talbot et al., U.S. Pat. No. 6,752,979, the disclosure of which is incorporated herein by reference, describes a method of producing metal oxide particles having nano-sized grains that consists of: mixing a solution containing one or more metal cations with a surfactant under conditions such that surfactant micelles are formed within the solution, thereby forming a micellar liquid; and heating the micellar liquid to remove the surfactant and form metal oxide particles having a disordered pore structure. The metal cations are selected from the group consisting of cations from Groups 1A, 2A, 3A, 4A, 5A, and 6A of the Periodic Table, transition metals, lanthanides, actinides, and mixtures thereof. Preparations of particles of cerium dioxide and mixed oxides containing cerium and one or more other metals are included in the illustrative examples.
Illustrative example 9 of U.S. Pat. Nos. 6,413,489 and 6,869,584, the disclosures of which are incorporated herein by reference, describes the inclusion of cerium nitrate in the emulsion mixture to prepare cerium-doped barium hexaaluminate particles, which were collected by freeze drying and calcined under air to 500° C. and 800° C. The resulting particles had grain sizes of less than 5 nm and 7 nm at 500° C. and 800° C., respectively. Illustrative example 10 describes the synthesis of cerium-coated barium hexaaluminate particles. Following calcination, the cerium-coated particles had grain sizes of less than 4 nm, 6.5 nm, and 16 nm at 500° C., 800° C., and 1100° C., respectively.
Wakefield, U.S. Pat. No. 7,169,196 B2, the disclosure of which is incorporated herein by reference, describes a fuel comprising cerium dioxide particles that have been doped with a divalent or trivalent metal or metalloid that is a rare earth metal, a transition metal, or a metal of Group Ha, IIIB, VB, or VIB of the Periodic Table. Copper is disclosed as a preferred dopant.
Oji Kuno in U.S. Pat. No. 7,384,888B2, the disclosure of which is incorporated herein by reference, describes a cerium-zirconium composite metal oxide with a ceria core and zirconia shell having improved high temperature stability and stable OSC. However, calcining at 700° C. is required for the preparation of the material, which shows a 10-20 percent improved catalytic activity with respect to hydrocarbon and carbon monoxide oxidation. No sizing data is provided to support the claim of 5-20 nm particles, no direct OSC measurements are quoted, and there is no analytical data to support the assertion of a core-shell geometry.
With regard to 10 nm diameter or smaller nanoparticles, there are multiple concerns that cast doubt on the ability of metal ion dopants to be incorporated in such small particles. For example, an 8.1 nm particle will have less than 10% of the Ce ions on the surface, whereas a 2.7 nm particle (5 unit cells on an edge of each 0.54 nm/unit cell) will have 46.6% of the 500 Ce ions on the surface. Surface ions are either ½ (for a face) or ⅛ (corner) incorporated into the lattice; therefore, their binding energies are substantially reduced and their coordination requirements unfulfilled. The difficulties associated with the doping of (semiconductor) nanocrystals is discussed in Science, 319, Mar. 28, 2008 by Norris et al. Characteristics such as the relative solubility of the dopant in the crystal vs solution, the diffusion of the dopant into the lattice, its formation energy, size and valence relative to the ions that are being replaced, kinetic barriers such as may be imposed by adsorbed surface stabilizers may all play a role in determining the extent, if any, to which a dopant metal ion may be incorporated into nano crystals of these dimensions.
It is clear from the references just described, that, the majority of the doping work has occurred at relatively large particle size (20 nm or so) and was carried out either by calcining the initial cerium-metal dopant mixture, or by micellar synthesis—a process that does not readily lend itself to large scale material production. In the work describing particles of a size less than 10 nm, the crystallographic form has not been established nor has conclusive evidence of incorporation been provided.
Thus there exists a need to readily incorporate a wide variety of metal dopant ions into the cerium sub-lattice of cubic CeO2 for very small nanoparticles (less than about 10 nm diameter) in a facile manner that does not require calcination (500 C or greater) and to unequivocally demonstrate incorporation as opposed to the production a separately nucleated population of dopant metal oxide grains. As single crystal particles of ceria are unique, so too would be a metal lattice engineered variant of ceria. Additionally, it would be desirable to produce large commercially available quantities of these materials in an economical manner and in a relatively short period of time.
A typical chemical reactor that might be used to prepare cerium dioxide includes a reaction chamber that includes a mixer (see, for example, FIG. 1 in Zhou et al. U.S. Pat. No. 7,025,943, the disclosure of which is incorporated herein by reference). A mixer typically includes a shaft, and propeller or turbine blades attached to the shaft, and a motor that turns the shaft, such that the propeller is rotated at high speed (1000 to 5000 rpm). The shaft can drive a flat blade turbine for good meso mixing (micro scale) and a pitched blade turbine for macro mixing (pumping fluid through out the reactor).
Such a device is described in Antoniades, U.S. Pat. No. 6,422,736, the disclosure of which is incorporated herein by reference. The described reactor is useful for fast reactions such as that shown by the equation below, wherein the product, AgCl, is a crystalline material having a diameter on the order of several hundred nanometers up to several thousand nanometers.AgNO3+NaCl→AgCl+NaNO3 
Cerium dioxide particles prepared using this type of mixing are often too large to be useful for certain applications. It is highly desirable to have the smallest cerium dioxide particles possible as their catalytic propensity, i.e., their ability to donate oxygen to a combustion system (cf. equation 1), increases with decreasing particle size, especially for particles having a mean diameter of less than 10 nm.
PCT/US2007/077545, METHOD OF PREPARING CERIUM DIOXIDE NANOPARTICLES, filed Sep. 4, 2007, describes a mixing device that is capable of producing CeO2 nanoparticles down to 1.5 nm, in high yield and in very high suspension densities. The reactor includes inlet ports for adding reactants, a propeller, a shaft, and a motor for mixing. The reaction mixture is contained in a reactor vessel. Addition to the vessel of reactants such as cerium nitrate, an oxidant, and hydroxide ion can result in the formation of CeO2 nanoparticles, which are initially formed as very small nuclei. Mixing causes the nuclei to circulate; as the nuclei continuously circulate through the reactive mixing regime, they grow (increase in diameter) as they incorporate fresh reactants. Thus, after an initial steady state concentration of nuclei is formed, this nuclei population is subsequently grown into larger particles in a continuous manner. Unless grain growth restrainers are employed to terminate the growth phase, this nucleation and growth process is not desirable if one wishes to limit the final size of the particles while still maintaining a high particle suspension density.
An example of this nucleation and growth process applied to the aqueous precipitation of CeO2 is the work of Zhang et al., J. Appl. Phys., 95, 4319 (2004) and Zhang, et al., Applied Physics Letters, 80, 127 (2002). Using cerium nitrate hexahydrate as the cerium source (very dilute at 0.0375M) and 0.5 M hexamethylenetetramine as the ammonia precursor, 2.5 to 425 nm cerium dioxide particles are formed in times less than 50 minutes. These particles are subsequently grown to 7.5 nm or greater using reaction times on the order of 250 minutes or 600 minutes, depending upon growth conditions. The limitations of particle size, concentration and reaction time would exclude this process from consideration as an economically viable route to bulk commercial quantities of CeO2 nanoparticles.
I. H. Leubner, Current Opinion in Colloid and Interface Science, 5, 151-159 (2000), Journal of Dispersion Science and Technology, 22, 125-138 (2001) and ibid. 23, 577-590 (2002), and references cited therein, provides a theoretical treatment that relates the number of stable crystals formed with molar addition rate of reactants, solubility of the crystals, and temperature. The model also accounts for the effects of diffusion, kinetically controlled growth processes, Ostwald ripening agents, and growth restrainers/stabilizers on crystal number. High molar addition rates, low temperatures, low solubility, and the presence of growth restrainers all favor large numbers of nuclei and consequently smaller final grain or particle size.
In contrast to batch reactors, colloid mills typically have flat blade turbines turning at 10,000 rpm, whereby the materials are forced through a screen whose holes can vary in size from fractions of a millimeter to several millimeters. Generally, no chemical reaction is occurring, but only a change in particle size brought about by milling. In certain cases, particle size and stability can be controlled thermodynamically by the presence of a surfactant. For example, Langer et al., in U.S. Pat. Nos. 6,368,366 and 6,363,237, the disclosures of which are incorporated herein by reference, describe an aqueous micro emulsion in a hydrocarbon fuel composition made under high shear conditions. However, the aqueous particle phase (the discontinuous phase in the fuel composition) has a large size, on the order of 1000 nm.
Colloid mills are not useful for reducing the particle size of large cerium dioxide particles because the particles are too hard to be sheared by the mill in a reasonable amount of time. The preferred method for reducing large agglomerated cerium dioxide particles from the micron size down into the nanometer size is milling for several days on a ball mill in the presence of a stabilizing agent. This is a time consuming, expensive process that invariably produces a wide distribution of particle sizes. Thus, there remains a need for an economical and facile method to synthesize large quantities, at high suspension densities, of very small nanometric particles of cerium dioxide having a uniform size distribution and incorporating one or more transition metal ions while still maintaining the CeO2 cubic fluoroite structure.
Aqueous precipitation may offer a convenient route to cerium nanoparticles. However, to be useful as a fuel-borne catalyst for fuels, cerium dioxide nanoparticles must exhibit stability in a nonpolar medium, for example, diesel fuel. Most stabilizers used to prevent agglomeration in an aqueous environment are ill suited to the task of stabilization in a nonpolar environment. When placed in a nonpolar solvent, such particles tend to immediately agglomerate and, consequently, lose some, if not all, of their desirable nanoparticulate properties. Thus, it would be desirable to form stable cerium dioxide particles in an aqueous environment, retain the same stabilizer on the particle surface, and then be able to transfer these particles to a nonpolar solvent, wherein the particles would remain stable and form a homogeneous mixture or dispersion. In this simplified and economical manner, one could eliminate the necessity for changing the affinity of a surface stabilizer from polar to non-polar. Changing stabilizers can involve a difficult displacement reaction or separate, tedious isolation and re-dispersal methods such as, for example, precipitation and subsequent re-dispersal with the new stabilizer using ball milling.
Thus, there remains a need for an efficient and economical method to synthesize stable transition metal-containing cerium dioxide nanoparticles in a polar, aqueous environment, and then transfer these particles to a non-polar environment wherein a stable homogeneous mixture is formed.
The use of cerium nanoparticles to provide a high temperature oxidation resistant coating has been reported, for example, in “Synthesis Of Nano Crystalline Ceria Particles For High Temperature Oxidization Resistant Coating,” S. Seal et al., Journal of Nanoparticle Research, 4, pp 433-438 (2002). The deposition of cerium dioxide on various surfaces has been investigated, including Ni, chromia and alumina alloys, and stainless steel and on Ni, and Ni—Cr coated alloy surfaces. It was found that a cerium dioxide particle size of 10 nm or smaller is desirable. Ceria particle incorporation subsequently inhibits oxidation of the metal surface.
Rim, U.S. Pat. No. 6,892,531, the disclosure of which is incorporated herein by reference, describes an engine lubricating oil composition for a diesel engine that includes a lubricating oil and 0.05-10 wt. % of a catalyst additive comprising cerium carboxylate.
As described above, currently available cerium oxide- and doped cerium oxide-based fuel additives have improved fuel combustion of diesel engines; however further improvements are still needed. It would be desirable to formulate these fuel additives for diesel engines that provide further improved fuel combustion by taking advantage of even smaller, sub 5 inn nanoparticles of cubic CeO2 with higher specific surface areas. The increased oxygen storage capability enabled by the inclusion of transition metals at these grain sizes is also highly desirable. In addition, protection of engines from wear, reduced engine friction, and greater lubricity, with simultaneously improved fuel efficiency would be tremendously beneficial.